The detailed mechanisms by which oxygenated diesel fuels reduce engine-out soot emissions are not well understood. The literature contains conflicting results as to whether a fuel's overall oxygen content is the only important parameter in determining its soot-reduction potential, or if oxygenate molecular structure or other variables also play significant roles.
To begin to resolve this controversy, experiments were conducted at a 1200-rpm, moderate-load operating condition using a modern-technology, 4-stroke, heavy-duty DI diesel engine with optical access. Images of broadband natural luminosity (i.e., light emission without spectral filtering) from the combustion chamber, coupled with heat-release and efficiency analyses, are presented for three test-fuels. One test-fuel (denoted GE80) was oxygenated with tri-propylene glycol methyl ether; the second (denoted BM88) was oxygenated with di-butyl maleate. The overall oxygen contents of these two fuels were matched at 26% by weight. The third test-fuel (denoted CN80) was a non-oxygenated, 80-cetane blend of two C16H34 primary reference fuels. The compositions of the three test-fuels were tailored such that each fuel had the same ignition delay at the given operating condition. Oxygen content, combustion phasing, and adiabatic flame temperature also were carefully matched to isolate (as much as possible) the effects of oxygenate molecular structure on combustion and soot-formation processes.
Whereas no dramatic differences in the spatial development of ignition or combustion processes among the fuels are evident from the natural luminosity (NL) images, spatially integrated natural luminosity (SINL) data, used as a relative measure of the average in-cylinder soot volume fraction, show differences among the fuels. The non-oxygenated CN80 produced 3 times and >7 times higher peak SINL than the oxygenated BM88 and GE80 fuels, respectively. The peak SINL measured for BM88 was twice as great as for GE80, indicating that overall oxygen content is not the only important parameter in determining the soot-reduction potential of an oxygenated fuel.